1. Field of the Invention
This invention relates to printed circuit boards, and more particularly, a method and apparatus for measuring mechanical strain in a printed circuit board.
2. Description of the Related Art
During the manufacturing of electronic assemblies including printed circuit boards (PCB's), various processes may introduce mechanical strains. Some processes, such as a wave solder operation or an IR (infrared) reflow operation may strain a PCB thermally and thus cause the PCB to expand. Other processes such as in-circuit test (ICT) may strain the PCB mechanically, as an ICT often times requires the assembly to be secured in a caged fixture that is operated by a vacuum. Other manual handling processes may also result in mechanical strain.
The mechanical strain placed on a PCB during the manufacturing process of an electronic assembly must be kept within certain limits. If these limits are exceeded, the assembly may become damaged. For example, solder joints for surface mounted components such as ball-grid arrays (BGA's) or components mounted in plated-through holes (PTH's). Signal lines can also be damaged if the strain exceeds limits.
Strain gages are used to monitor the strain placed on a PCB during the manufacturing process of an electronic assembly. A strain gage is a device that is mounted to a PCB in a location at or near where the strain is to be measured. FIG. 1 is a drawing of two exemplary embodiments of a strain gage. Strain gages such as those illustrated in FIG. 1 include one or more conductive paths. The use of strain gages is based upon the principle that the resistance of a conductor will change when it is subjected to strain. Typically, when a conductor is stretched, it becomes longer and narrower, and therefore its resistance increases. Resistance measurements may be taken before, during, and after the strain gage is subjected to strain. These resistance measurements may be used to determine the strain encountered by the strain gage, and may also be used to determine the rate of strain. Resistance measurements are made by attaching resistance-measuring test equipment (e.g., a multi-meter) to the wire leads of the strain gage.
As noted above, strain gages may be mounted to a PCB in order to determine the amount of strain to which the PCB is subjected during the manufacturing process of an electronic assembly. FIG. 2 illustrates the mounting of strain gages on a PCB in the vicinity of a BGA footprint on the PCB. In the embodiment shown, strain gages are mounted on the PCB near each of the corners of a BGA footprint. More particularly, the strain gages may be attached to the PCB by the use of epoxy. Resistance measurements taken for each of the mounted strain gages may allow the approximation of the strain encountered on the PCB by the portion upon which the BGA is mounted. However, the locations of these strain gages, while close to the BGA footprint, are not ideal. Various factors, such as the thermal characteristics of the BGA solder balls, may limit the effectiveness of strain measurements at these points. Furthermore, the electrical conductors of typical strain gages (such as that shown in FIG. 1) are typically encased in a film-like material which has different characteristic than the PCB. Such materials may affect the amount the electrical conductor is stretched when placed under a process that induces strain. Another factor that must be considered to ensure accurate measurements is matching the coefficient of thermal expansion (CTE) of the strain gages with that of the electronic assembly of which the PCB is a part. Failure to match the CTE's with sufficient precision can further limit the accuracy of strain measurements.
Thus, with limited accuracy, the strain measurements taken at these locations may not yield a true picture of the strain encountered on the PCB at the BGA footprint. In embodiments where strain gages are attached using an epoxy, only a small sample of PCB's may be used for strain measurements.